Screw vacuum pump with male and female screw rotors having unequal leads

ABSTRACT

In a screw vacuum pump  30 , equal lead screws each in the range of 1 to 4 leads are added on the discharge side of male and female screw rotors ( 4 ) and ( 5 ) having a continuously changing screw gear helix angle, at a final lead angle of the male and female unequal lead screw rotors on the discharge side that continuously change in helix angle following the advance of the screw gear helix.

TECHNICAL FIELD

This invention relates to a screw vacuum pump and, in particular,relates to a screw vacuum pump that is optimal for a region fromatmospheric pressure to 0.1 Pa.

BACKGROUND ART

In a semiconductor device manufacturing system, since a serious problemarises in a semiconductor device manufacturing process if oil backdiffusion occurs from a pump into a process chamber of the semiconductordevice manufacturing system, use has conventionally been made of aso-called dry pump, a mechanical booster pump, a turbomolecular pump,and the like where there is no occurrence of contact between suction gasand oil.

With respect to these dry pump, mechanical booster pump, and screw pump,a problem exists that shaft seals are provided at both ends, i.e. on thesuction side and the discharge side, and particularly a seal gas amountof the shaft seal on the suction side and a leakage amount from the sealcause a reduction in pumping speed so that there is no alternative butto use such a pump that has an unnecessarily high pumping speed.

Further, since molecular weights of process gas, carrier gas, gas to beproduced, and so on are broad, i.e. from 1 to one hundred and severaltens, it is the current situation that the foregoing pumps are properlyused depending on their pumping characteristics for those various gasesand their inherent pumping regions.

On the other hand, a problem exists that since the pumping speed islowered depending on the kind of exhaust gas, a pump having a largepumping speed is inefficiently used. Further, with respect to generaldry pumps and mechanical booster pumps, there is a problem that productis deposited inside the pump between an inlet port and a discharge port.

Drawbacks of a conventional screw pump will be explained with referenceto FIG. 5.

Referring to FIG. 5, in the conventional screw vacuum pump, since aback-diffusion amount from a discharge port and a back-diffusion amountof diluent gas are large, the ultimate pressure becomes about 3 Pa and,as indicated by a curve 2 in FIG. 5, the pumping speed largely decreaseson the molecular flow region side. Further, the pumping speed forhydrogen becomes ⅓ to ½ of that for nitrogen and, as indicated by acurve 3 in FIG. 5, since the compression ratio is small, the pumpingspeed extremely decreases.

Further, since screw engagement of the conventional screw vacuum pump isnot located outside gear engagement pitch circles determined by adistance between axes of male and female rotors and the numbers of teethof the male and female rotors, product generated in the semiconductordevice manufacturing process is stuck to screw engagement portions,thereby causing failure.

Therefore, it is an object of this invention to provide a screw vacuumpump that can maintain the stable pumping performance down to about 0.1Pa regardless of the kind of gas.

DISCLOSURE OF THE INVENTION

For accomplishing the foregoing object, according to one aspect of thepresent invention there is provided a screw vacuum pump which comprisesa male rotor and a female rotor respectively having engagement screwgears, a stator receiving therein both rotors, a gas working chamberformed by the male rotor and the female rotor and the stator, and aninlet port and a discharge port for a gas provided at the stator so asto be capable of communicating with one end portion and the other endportion of the working chamber, respectively. In the aspect of thepresent invention, the male and female rotors each comprise a main leadscrew in which a helix angle of the screw gear continuously changesfollowing the advance of helix, and an auxiliary lead screw in the formof an equal lead screw formed in the range of 1 to 4 leads at a finallead angle of the main lead screw on a discharge side of the male andfemale rotors.

In the aspect of the present invention, by forming the equal leadportions having the lead angle equal to the discharge-end lead angle onthe discharge side, it is possible to prevent back diffusion of the gasand largely improve the compression ratio and, as a result of preventingthe back diffusion, it is possible to reduce the consumption power andreduce the heat that is generated on the discharge side.

Further, according to another aspect of the present invention there isprovided a screw vacuum pump which comprises a male rotor and a femalerotor respectively having engagement screw gears, a stator receivingtherein both rotors, a gas working chamber formed by the male rotor andthe female rotor and the stator, and an inlet port and a discharge portfor a gas provided at the stator so as to be capable of communicatingwith one end portion and the other end portion of the working chamber,respectively. In the aspect of the present invention, the male andfemale rotors each comprise a main lead screw in which a helix angle ofthe screw gear continuously changes following the advance of helix, andan additional lead screw provided on an inlet side of the male andfemale rotors, the additional lead screw being in the form of an equallead screw formed in the range of 0.2 to 1 lead at a lead angle of themain lead screw at its end portion on the inlet side.

Further, according to still another of the present invention, there isprovided a screw vacuum pump which comprises a male rotor and a femalerotor respectively having engagement screw gears, a stator receivingtherein both rotors, a gas working chamber formed by the male rotor andthe female rotor respectively having engagement screw gears and thestator, and an inlet port and a discharge port for a gas provided at thestator so as to be capable of communicating with one end portion and theother end portion of the working chamber, respectively. In the aspect ofthe present invention, engagement of the screw gears of the male andfemale rotors is located outside gear engagement pitch circlesdetermined by a distance between axes of the male and female rotors andthe numbers of teeth of the male and female rotors.

Further, according to yet another aspect of the present invention, thereis provided a screw vacuum pump which is characterized in that the screwgears of the male and female rotors have mutually different numbers ofteeth in any one of the foregoing screw vacuum pumps.

Further according to a further aspect of the present invention, there isprovided a screw vacuum pump which is characterized in that the male andfemale rotors each comprise an unequal lead screw at its middle portionin an axial direction thereof in any one of the foregoing screw vacuumpumps.

Further, in a screw vacuum pump according to the aspect of the presentinvention, equal lead portions having a lead angle equal to aninlet-side lead angle and equal lead portions having a lead angle equalto a discharge-end lead angle are formed on an inlet side and adischarge side, respectively, of male and female unequal lead screwrotors, and engagement of the male and female screw rotors is formed ata position outside gear engagement pitch circles determined by adistance between axes of the male and female rotors and the numbers ofteeth of the male and female rotors. Therefore, it is possible toincrease the compression ratio, obtain an effect of raking out product,and maintain the stable pumping speed down to 0.1 Pa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the overall structure of a screwvacuum pump according to an embodiment of this invention;

FIG. 2 is an expansion view wherein equal lead screws are added tounequal lead screws in FIG. 1 according to the embodiment of thisinvention;

FIG. 3 is a development view on base cylinders according to theembodiment of this invention, wherein tooth helix curves of tooth-shapedexternal contact portions in the form of parabolas (quadratic curves)are shown on a coordinate axis in which the axis of abscissa representsmale and female rolling circumferential lengths of the base cylindersand the axis of ordinate represents a helix advancing amount;

FIG. 4 is a perpendicular-to-axis sectional view of the screws accordingto the embodiment of this invention; and

FIG. 5 is a diagram showing a comparison in pumping speed between thepump according to this invention and a conventional pump.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, an embodiment of this invention will be described withreference to FIGS. 1 to 4.

FIG. 1 is a sectional view showing the overall structure of a screwvacuum pump according to the embodiment of this invention, wherein maleand female screw rotors 4 and 5 are illustrated as having unequal leads.FIG. 2 is an expansion view showing a structure in which equal leadscrews are added to unequal lead screws, respectively, of the male andfemale screw rotors shown in FIG. 1 according to the embodiment of thisinvention, FIG. 3 is a development view showing tooth rolling curves ofthe lead screws according to the embodiment of this invention, and FIG.4 is a diagram showing a relationship between engagement of the male andfemale screws and engagement circles determined by a distance betweenthe male and female axes and the numbers of teeth of the screws.

This invention will be described in further detail. As shown in FIG. 2,on the discharge side of unequal lead portions (main lead screws) 4 band 5 b of the male and female unequal lead screw rotors 4 and 5, equallead portions (sub lead screws) 4 c and 5 c having a lead angle equal toa discharge-end lead angle of the unequal lead portions 4 b and 5 b areformed in the screw vacuum pump.

In this invention, as shown in FIGS. 2 and 3, the equal leads 4 c and 5c each having 1 to 4 leads are added at the discharge end of the unequallead screws as equal leads having the discharge-end lead angle of theunequal lead screws. In the unequal lead screws, the tooth helix angleof screw gears forming male and female rotors of an unequal lead screwvacuum pump changes according to a rotation angle of the rotors tothereby change the volume of a V-shaped working chamber formed by therotors and a stator. In the unequal lead screw vacuum pump, a workingchamber with a constant volume that merely transfers a sucked gaswithout compressing it is abolished and the volume between the leads ofthe engagement female and male lead screw rotors is continuously reducedso that all working chambers serve to compress the gas.

An important point of this invention herein resides in that thecompression ratio of the screw vacuum pump is reduced by adding theequal leads 4 c and 5 c at the discharge end, thereby suppressing backdiffusion from a discharge port 10 shown in FIG. 1. This is because aback-diffused gas enters the working chamber and is again compressed andexhausted, thereby increasing an electric power consumption and, due toan increase in back diffusion, the ultimate pressure and the pumpingspeed are largely affected. The suppression of the back diffusion leadsto electric power saving.

Further, since the unequal lead screws perform compression and exhausteven at their final lead portions, expansion and deformation occur dueto compression heat near the discharge port to thereby cause contactbetween the screws and between the screw and the stator, which is thusnot preferable. This invention solves this problem by adding the equalleads 4 c and 5 c having a thermally stable structure and a structurethat facilitates precise processing.

Further, in this invention, as shown in FIG. 4, it is configured thatengagement of the male and female screw rotors 21 and 22 of the screwvacuum pump is located outside gear engagement pitch circles 15 and 16determined by a distance between the axes of the male and female rotorsand the numbers of teeth of the male and female rotors, therebyproviding no tooth surfaces where the tooth-surface speeds of the maleand female screws are equal to each other, to obtain the state where afaster tooth surface slides on a slower tooth surface, to therebyachieve an operation of raking out sucked reaction product or the likeexisting between the tooth surfaces and thus achieve an effect of rakingout the reaction product to the exterior of the pump.

Now, one example of the screw vacuum pump according to this inventionwill be described in further detail with reference to FIGS. 1 to 4.

Referring to FIG. 1, a screw vacuum pump 30 has a structure in which afirst housing 31, a second housing 32, and a third housing 33 areconnected in an axial direction in the order named from the pump side.

The first housing 31 comprises a stator 13 and has one end side providedwith an inlet port 14 for sucking a fluid and the other end sidecommunicating with the second housing 32. At a connecting portion, withthe first housing 31, of the second housing 32, the discharge port 10 isprovided for discharging the fluid. In the stator 13 of the firsthousing 31, a female screw rotor 4 and a male screw rotor 5 are disposedthat mesh with each other and use, as their rotation shafts, a firstshaft 23 and a second shaft 24 received in the second housing 32.

In the second housing 32, the first shaft 23 serving as the rotationshaft of the female screw rotor 4 and the second shaft 24 serving as therotation shaft of the male screw rotor 5 are provided so as to extend inthe axial direction from the respective screw rotors 4 and 5 disposed inthe first housing 31, and the first shaft 23 extends into the thirdhousing 33. The first shaft 23 and the second shaft 24 are rotatable bythe use of bearings 9 disposed at both ends of the respective shafts inthe second housing 32. An oil splashing mechanism 11 is disposed aroundthe second shaft 24 in the second housing 32 and engagement timing gears12 are provided at substantially the same positions in the axialdirection of the first shaft 23 and the second shaft 24.

In the third housing 33, an electric motor 8 is disposed which uses oneend of the first shaft 23 as its rotation shaft. The first shaft 23 heldby the bearings 9 is rotated by the motor 8 disposed in the thirdhousing 33 and this rotation synchronously rotates the first and secondshafts 23 and 24 through the timing gears 12. The oil splashingmechanism 11 is attached to the second shaft 24 for supplying oil to thetiming gears 12 and the bearings 9.

On the pump side, a high vacuum is achieved by high-speed rotation ofthe screw rotors comprising the female screw rotor 4 and the male screwrotor 5.

Referring to FIG. 2, the male screw rotor is formed by an equal leadscrew 5 a, the unequal lead screw 5 b, and the equal lead screw 5 c fromthe suction side. Likewise, the female screw rotor is formed by an equallead screw 4 a, the unequal lead screw 4 b, and the equal lead screw 4 cfrom the suction side. In this invention, additional lead screwsrepresent the equal lead screws 4 a and 5 a. Specifically, in thisembodiment, the outer diameter of the male screw rotor is set to 80 mmand the inner diameter of the female screw rotor is set to 100 mm. Theequal lead screws 4 a and 5 a and the equal lead screws 4 c and 5 c onthe suction and discharge sides are each set to a length of about 50 mmand may be set in the range of 0.2 to 1 lead and in the range of 1 to 4leads, respectively. When being outside these ranges, each of equalleads screws 4 a and 5 a has a less effect of thermally stableoperation. It is preferable that the lead angle of the equal lead screws4 a and 5 a on the suction side be set to 45 degrees where the maximumefficiency is obtained. Further, the unequal lead screws 4 b and 5 b atthe middle portion are each set to a length of about 120 mm.

FIG. 3 is a development view showing tooth rolling curves in the form ofparabolas (quadratic curves) on a coordinate axis in which the axis ofabscissa represents male and female rolling circumferential lengths ofthe base cylinders and the axis of ordinate represents a helix advancingamount, wherein the male screw rotor 5 comprises an equal lead screw 5 a1, an unequal lead screw 5 b 1, and an equal lead screw 5 c 1 from thesuction side and the female screw rotor 4 comprises an equal lead screw4 a 1, an unequal lead screw 4 b 1, and an equal lead screw 4 c 1 fromthe suction side.

Next, FIG. 4 is a perpendicular-to-axis sectional view of the male andfemale screws. As shown in FIG. 4, the number of teeth of the male screwrotor 5 is smaller than that of the female screw rotor 4. On the otherhand, a male screw outer diameter 19 is larger than a female screw outerdiameter 20. By providing the engagement between male screw teeth 21 andfemale screw teeth 22 outside the gear engagement pitch circle 15 of themale screw rotor 5 and the gear engagement pitch circle 16 of the femalescrew rotor 4 according to the foregoing conditions, the tooth-surfacespeeds of the male and female screws differ from each other so that thetooth surfaces of the male screw teeth 21 slide on the tooth surfaces ofthe female screw teeth 22 to rake out the product or the like existingbetween the tooth surfaces of the male and female screw teeth.

As described above, in the embodiment of this invention, the pumpingspeed of the screw vacuum pump is largely improved as indicated by acurve 1 in FIG. 5 so that the stable pumping speed can be achievedefficiently from the atmospheric pressure to 0.1 Pa by the use of onlyone vacuum pump, thereby covering the wide operation range. Further, theeffect has been achieved that rakes out the reaction product.

As described above, according to this invention, the effect can beachieved that the pumping speed of the screw vacuum pump is largelyimproved so that the stable pumping speed can be obtained efficientlyfrom the atmospheric pressure to 0.1 Pa by the use of only one vacuumpump, thereby covering the wide operation range. Further, in thisinvention, the effect can be achieved that rakes out the reactionproduct.

Further, by the use of the vacuum pump of this invention, an effect canbe achieved that it is possible to constitute a vacuum system that issimple in structure and low in price as compared with a vacuum system inthe combination of the conventional dry pump, mechanical pump, and soon.

Moreover, according to this invention, since the structure of the vacuumsystem becomes simple, an effect can be achieved that complicatedoperations such as switching of valves become unnecessary to therebyenable a control system to be simple and low-priced.

INDUSTRIAL APPLICABILITY

As described above, the screw vacuum pump according to this invention issuitable as a vacuum pump for use in a system of manufacturingsemiconductor devices and so on.

1. A screw vacuum pump comprising a pair of male and female unequal leadscrew rotors, wherein said male and female unequal lead screw rotorsinclude equal lead portions having a lead angle equal to an inlet-sidelead angle and formed on an inlet side thereof; equal lead portionshaving a lead angle equal to a discharge-end lead angle and formed on adischarge side thereof; unequal portions having a lead anglecontinuously decreasing from the inlet-side lead angle to thedischarge-end lead angle and formed between the equal lead portionsformed on the inlet side and the equal lead portions formed on thedischarge side; and engagement between the male and female lead screwrotors is formed at a position different from each of gear engagementpitch circles, said circles being determined by a distance between axesof said male and female rotors and the numbers of teeth of engagementgears of said male and female rotors, each of said unequal lead portionshaving a length longer than that of each of said equal lead portionsalong an axis of said pair of male and female rotors.
 2. A screw vacuumpump comprising a male rotor and a female rotor respectively havingengagement screw gears, a stator receiving therein both rotors; a gasworking chamber formed by the male rotor and the female rotor and thestator; and an inlet port and a discharge port for a gas provided atsaid stator configured to communicate with one end portion and the otherend portion of said working chamber, respectively, wherein each of saidmale and female rotors includes a main lead screw at its middle portionin which a helix angle of said screw gear continuously changes followingthe advance of helix, a sub lead screw in the form of an equal leadscrew formed on a discharge side of said male and female rotors, and anadditional lead screw in the form of an equal lead screw provided on aninlet side of said male and female rotors, said main lead screw having alength longer than that of each of said sub lead screw and saidadditional lead screw along an axis of each of said male and femalerotors.
 3. The screw vacuum pump according to claim 2, wherein the sublead screw is in the form of the equal lead screw formed in the range of1 to 4 leads at a lead angle of said main lead screw.
 4. The screwvacuum pump according to claim 2, wherein said additional lead screw isin the form of the equal lead screw formed in the range of 0.2 to 1 leadat a lead angle of said main lead screw.
 5. The screw vacuum pumpaccording to any one of claims 2 to 4, wherein engagement of said screwgears of said male and female rotors is located at a portion differentfrom each of gear engagement pitch circles determined by a distancebetween axes of said male and female rotors and the numbers of teeth ofsaid male and female rotors.
 6. The screw vacuum pump according to anyone of claims 2 to 4, wherein each of said screw gears of said male andfemale rotors have mutually different numbers of teeth.
 7. The screwvacuum pump according to claim 1, wherein said inlet-side lead angle is45 degrees.
 8. The screw vacuum pump according to claim 2, wherein saidadditional lead screw has a lead angle of 45 degrees.